1. Field of the Invention
The present invention relates to load insulators, and particularly to insulated links for interposition between a lifting device, such as an overhead crane, and a load, so as to electrically isolate the load from the lifting device.
2. History of the Prior Art
Where heavy loads are lifted by cranes or other similar mobile lifting apparatus, a particular danger exists from overhead power lines. If a part of a crane contacts a power line, a worker who is in electrical contact with the load, such as a rigger who may be controlling the lateral movement of the load, for example by means of ropes, or may himself be in physical contact with the load, may be in danger of severe electrocution as current passes to the earth. To prevent this from happening, it is known to interpose a load insulator into the loaded line. Typically, load insulators are comprised of a body and oppositely disposed fastening lugs for attachment to the lifting apparatus and to a load or loaded line. The body is load bearing in the sense that it is capable of supporting the substantial loads during the lifting operation. At the same time, and because the fastening lugs at the opposite ends of the body are typically made of steel or other conductive material, the body of the load insulator must be capable of electrically insulating the opposite lugs from each other.
Examples of load insulators are provided by U.S. Pat. No. 5,568,132 of Hugh M. Pratt, issued Oct. 22, 1996 and entitled "Load Insulator", and by a copending application of Hugh M. Pratt, Ser. No. 08/687,156, filed Jul. 24, 1996 and entitled "Load Insulator". The '132 patent and the '156 application describe load insulators in which mechanical links within a tubular body couple opposite links which form lugs at opposite ends of the body in chain-like fashion. At the same time, at least one insulative insert is interposed between an adjacent pair of the links within the body or on the outside of a load pin coupling an adjacent pair of the links within the body, to provide the necessary electrical insulation. The mechanical links of the body and the insulative insert are contained within a tubular housing member in which foam-like material has been inserted to restrict movement of the links and the insert.
As in the case of the load insulators described in the '132 patent and the '156 application, the body of the load insulator is either finished in or is covered by a layer of plastic or other material which electrically insulates the outside of the body and tends to prevent rain water and other moisture from entering. At the same time, the fastening lugs at the opposite ends of the body are not insulated but are exposed to the outside of the load insulator. This has been found to produce several problems. If the load line comes in contact with high voltage so that a large current must pass through the load insulator, ideally most or all of the current passes through the inside of the load insulator body where the electrical insulation is substantial. However, in cases of very high voltage, at least some of the current tends to creep around or flash over the outside of the body between the upper and lower lugs. The amount of current which flows over the outside of the load insulator has been observed to increase with the moisture level and the degree of contamination at the outer surface of the insulator body. Substantial currents bypassing the highly insulative interior of the body by flowing over the outer surface thereof increase the danger of electrocution of an operator working with the load.
A further problem with presently known insulators relates to the fact that the insulative outer surface or covering of the body terminates at the opposite end lugs. The interfaces there between tend to wick rain water or other moisture to the interior of the body, with harmful effects to the load insulator and to its insulative properties.
It would therefore be desirable to provide a load insulator which greatly minimizes or eliminates the flow of current over the outside of the insulator body between the opposite lugs. It would also be desirable to provide a load insulator having the exterior thereof sealed against rain water and other moisture so as to be less susceptible to water percolation through the insulator.